Odds of Hanging On
To remain in or excel at this job you're going to have to focus some of your energies on two words that might really scare you right now: continuing education.
Sure, you have firefighting experience, a fancy degree, and an affinity for soot, but that's just the basics of fire investigation. To improve your abilities (not to mention your cash intake), you'll want to get certified with any of the recognized fire professional associations around the country.
For instance, the National Association of Fire Investigators offers credentials in fires and explosives, investigation instruction, and another certification solely related to fires involving vehicles (source). And you don't just earn these once; every five years you're going to have to prove it all over again.
So yeah, continuing education. Just deal with it—the extra cash should ease your pain.